X-linked anhidrotic ectodermal dysplasia (EDA) is the most frequently occurring of more than 175 ectodermal dysplasias affecting one or more skin appendages. Individuals with EDA have sparse hair, rudimentary teeth, few sweat glands, and the gene is correspondingly involved at an early point in development. Although it is thus first seen as a pediatric disorder, it is also relevant to phenotypes that are aging-related, including the significant aging-related difficulty in tear production (dry eye) which is especially frequent in EDA patients and in thermoregulation. It is also relevant to the largely cosmetic but often worrying loss of hair (especially in premature baldness), for which the EDA pathway presents a conceivable route to stem cell-based regeneration. We demonstrated that the gene mutated to cause this disorder encodes a protein that we have named ectodysplasin-A. It has a single transmembrane region with collagenous and TNF-ligand segments in a long extracelliular carboxyterminal tail. In a search to obtain a useful laboratory model, we discovered that the Tabby mouse, which has many of the features observed in human EDA, is specifically mutated in the corresponding mouse gene. We found that provision of DNA encoding a variant of ectodysplasin in embryonic mice restores hair follicles and sweat glands. We also have characterized eye phenotypes of Tabby mice including blindness and inflammation susceptibility, and they are also reversed by supplementation with the same isoform. Those studies have provided the first animal model for ocular surface disease, and also further increase the interest in the possibility of manipulating the EDA pathway to combat dry eye. By generating a conditional transgenic mouse model, we further revealed and analyzed spatiotemporal actions of Eda-a1 during hair follicle development. For example, we found that Eda-a1 is needed only for induction and initial progression stages, but not for late differentiation stage of hair follicles. We also showed that Eda-a1 was able to partially restore secondary hair including zigzag subtype in Tabby mice. We also found that Dkk4, a soluble Wnt antagonist, discriminates an Eda-independent mechanism of secondary hair follicle formation. It was a long-standing puzzle that Tabby mice lack primary hair, but produce large numbers of secondary hair. We revealed that while Eda controls primary hair follicle development, Dkk4 exclusively regulates secondary hair follicle formation. Notably, both pathways converge at the activation of downstream Shh. Finally, we completed genome-wide expression profiling during sweat gland development. We found that Shh and Foxa1 are likely key effectors for sweat gland development under Eda regulation. To further dissect the developmental mechanisms of skin appendage formation, we recently carried out two projects dealing with the role of Shh and Foxa1. First, we followed up our earlier findings that Shh is a downstream target of Eda during hair follicle development to examine features of its mechanism of action. By generating skin-specific Shh transgenic and knockout mice in the Eda-deficient Tabby mice, we demonstrated that Shh is required for elongation of Tabby hair shafts, but not for the induction of the primary hair follicles that are missing in Tabby mice. A manuscript is in press in Cell Cycle on these data. Second, in a project in the final stage of completion, we further analyzed the function of FoxA1 in sweat gland development and homeostasis by generating skin-specific FoxA1 knockout mice. The FoxA1 knockout mice showed striking anhidrosis, with abundant accumulation of glycoproteins in the lumens and ducts of otherwise complete sweat glands. By expression profiling, we found that an anion channel protein, Best2, was dramatically down-regulated in the FoxA1 knockout mice. We further found that Best2 knockout mice show severe hypohidrosis/anhidrosis, demonstrating that Best2 is a major functional target of FoxA1. Our data reveal a FoxA1-Best2 cascade as a fundamental genetic pathway in sweat glands, regulating sweat secretion.